Defining your goal
The goal of your go to market may be dependent on a few factors such as market/user appetite, your rollout plan and product availability per region. Always adjust based on your market and user knowledge.
Baseline segmentation criteria you can use:
Growth (existing users): Depending on your user base and technical setup you can choose to go to market first with a few existing users and keep a short feedback loop on their adoption experience and performance. The quantitative (e.g. performance) and qualitative (e.g. satisfaction) data collected from existing users can serve as a good anchor for your messaging when you decide to attract net new users with your payments proposition afterwards. It also helps to fine-tune your proposition for each target segment and helps quantifying user benefits.
Acquisition (net new users): If you believe announcing your payments offering to the world will lead you to grab a foothold in a new market, increase awareness or create a timely competitive advantage (as examples) then an acquisition play might be the best option. In this case you’ll likely need to explain how uniquely positioned your platform is in solving users' core needs and how an integrated payments experience plays a vital role in that.